1. Field of the Invention
This invention relates to a method and apparatus for effecting vibration of a test specimen. In particular, this invention relates to a method and apparatus for applying a controlled electromagnetic force to a turbine bucket to induce vibration of the bucket over a predetermined range of frequencies.
2. Description of the Prior Art
Numerous systems have been developed for vibration of test specimens to determine such specimen characteristics as natural resonant frequencies and resistance to fatigue failure. One of the most useful techniques involves utilizing an electromagnetic field to induce vibration since physical attachment to the specimen, which might alter characteristics to be determined or limit testing to static specimens, is not required. This is generally accomplished by exciting an electromagnet at a desired frequency and locating the specimen in the field generated thereby. To date, however, systems employing this technique have not effected control over force applied to the specimen to the degree necessary to optimize obtainable information.
Although previous techniques employing control of applied force in response to signals from force measurement sensors are known, these systems are limited to use at a single frequency or at a slowly changing frequency such as the primary natural resonant frequency of the specimen, which changes slowly as the specimen fatigues.
Previous systems are limited not only in the amount of useful information obtainable at a single frequency, but also in the amount of force available to be applied. The amount of force available is a function of electromagnet current and this current tends to decrease as frequency is increased when the power source is a typical electronic power amplifier, because of an accompanying increase in inductive reactance.
The apparatus of the present invention overcomes all of the aforementioned limitations by providing controlled application of electromagnetic force to a test specimen over a wide range of frequencies. Applied force is maintained at a desired level at any particular frequency within this range or as the frequency rapidly changes within this range. Control of applied force is maintained without the necessity of placing sensor devices in contact with the specimen, although sensor devices (e.g., accelerometers) may be attached to obtain information in addition to that derived from non-contacting sensor devices. Further, the available force has been maximized by inclusion of circuitry which automatically nullifies electromagnet inductive reactance at any frequency within the range of operation.